1. Field of Invention
This invention relates to optical filter assemblies providing protection from physical and/or environmental damage to optical components, and more particularly to optical filter assemblies providing protection to thin film filter components.
2. Related Art
Optical thin film filters (also referred to herein simply as “thin film filters”) allow selective transmission of bands of electromagnetic radiation. Optical thin film filters may provide high pass, low pass, or band pass functionality using any of a number of different thin film structures. For example, thin film filters are commonly achieved by depositing one or more refractory oxide materials or elemental materials such as silver or gold onto a transparent substrate (e.g., a plate of glass).
While thin film filters have many desirable qualities, they are sensitive to many physical and environmental sources of degradation or permanent damage. For example, thin film filters may be degraded or damaged by physical contact, humidity, and abrupt temperature changes.
Conventional optical filter assemblies for protecting thin film filters from physical and environmental degradation have included bonding a protective glass plate to the outer surface of the thin film filter, or depositing a durable coating on the outer surface of the thin film filter. However, these solutions have numerous drawbacks, including wavefront distortion and/or reduction in optical efficiency due to absorption or scattering of the electromagnetic radiation by the protective material. Furthermore, conventional optical filter assemblies have resulted in increased material and labor costs, and reduced manufacturing yield due to increased handling and processing of the optical filter assembly.
Optical filter assemblies employing protective glass plates have been constructed by bonding a glass plate to a thin film filter using an adhesive that is applied on the top surface of a thin film filter. Typically, to reduce reflective losses in such systems, an adhesive is selected to be index-matched to the glass cover, and is applied over a region including the light transmitting region of the thin film filter such that the adhesive forms a continuous contact with both the thin film filter and the glass cover.
Additional drawbacks of optical filter assemblies using such adhesives include that the adhesive may shrink or expand over time thus providing a source of wavefront distortion. Such degradation of adhesives may be exacerbated by photochemically-induced changes in the adhesive, or photothermally-induced degradation caused by absorption of electromagnetic radiation transmitted through the adhesive. In addition to wavefront distortions and reductions in optical efficiency discussed above, many adhesives exhibit fluorescence in the presence of electromagnetic radiation. Such fluorescence introduces electromagentic noise into an optical system.
FIG. 1 is a cross-sectional side view of an example of a conventional protective filter assembly 100. In filter assembly 100, instead of bonding a protective material (e.g., a glass plate or a durable coating) to a thin film filter or substrate, a sealed region 155 enclosing a thin film filter 114 is formed by a soldering process. The edges of substrates 110, 120 are deposited with metal in areas 122 and coupled to carrier 140 by a solder 132 to form sealed region 155. A spacer 130 is placed between the substrates 110, 120 to maintain separation between the substrates. While soldered filter assembly 100 has advantages over other conventional filter assemblies, the added deposition and soldering processes add manufacturing steps and costs.